Gene detectives trace human evolution in India, find third gene trail

There was a third unknown hominid element – like the
Neanderthals or Denisovans – who left some of their genes among ancestors of
present day Indians, besides known contributions from the other two established
ancestral hominid groups that lived in Europe and Central Asia, new research
suggests1.

A group of international geneticists suggests that this
third hominid group most likely inhabited the Indian neighbourhood before going
extinct around 50,000 years ago. Genetic evidence points to this unknown
hominid group being spread across the oceanic islands from Andaman and Nicobar
to Australasia.

Scientists from National Institute of Bio Medical Genomics
(NIBMG), Kalyani, West Bengal, India along with collaborators from Spain, UK,
Netherlands and China found this novel third gene trail while analysing human
genome samples from the Indian subcontinent. They attempted to trace back the history
of the region through 1000 whole genome samples, including mitochondrial DNA, using
high throughput techniques and advanced statistical methods. They found small
fractions of gene sequences that did not match with modern human genome samples
or with the existing traces of the other two hominid groups.

Majumder and his team had earlier analysed the Indian population
to detect five distinct ethnicities, contrary to reports that identified two major ethnic
groups2. “With more representative population sampling, dissecting
the genome and rigorous statistical analysis, we identified two more ancestral
lineages in Indian population as Ancestral Austro-Asian (AAA) and Ancestral
Tibeto-Burman (ATB),” Majumder told Nature
India. The study also found that the ancestry of population in the Andaman
archipelago is distant from mainland Indians and closer to oceanic islanders of
the region.

This led the gene detectives to the third hominid group, possibly
inhabiting South-east Asia and Australia around 60,000 years ago and later, and
predicted to have emerged from the African hominid stock over 300,000 years
ago. The scientists suggest the probability of this group mixing with human
stocks around 50,000 – 60,000 years back and ultimately replacing it. In true
sense those earlier hominid inhabitants were not separate species, but sub-species
of the modern ancestral stock that came out of Africa around 80,000 years ago
to evolve as Homo sapiens sapiens of today.

The NIBMG scientists traced more or less similar amount of
Neanderthal genes in Onges and Jarawas as in the mainlanders. This suggests that
the admixture with Neanderthals occurred early in the primitive stock of
migrating ancestral human populations.

Interestingly, the third gene trail also highlights the
reason behind ethnic diversity in India. It urges a rethink on the route of
dispersal of humans and the process of colonization in south-east Asian lands
and Australia where India might have played an important transit point.

The study strengthens the ‘southern exit route’ of the ‘Out
of Africa’ (OOA) theory which says that part of a hominid population from east
Africa dispersed into different parts of Europe and Asia over 80,000 years
back. Many went north to northern Eurasia, then migrated to central Asia (northern
exit route) and may have gradually trickled back to southern Asia later. They
encountered Neanderthals and Denisovans on this path. Another group took a
southward route to enter Asia through Asia Minor, Arabian deserts and Hindu
Kush mountains to the plains of India. They started their journey to south-eastern
oceanic islands including the Australian land mass (southern exit route). In this
path, Andaman and Nicobar islands was a transit point. The 2-3% mixing of
unknown hominid genes in the Andaman islanders indicates the possibility of
modern human invasion into an inhabited territory around 60,000 years ago.

“There might have been waves of human invaders through the
southern route and Onges or Jarawas are probably the remnants of the earliest
wave who had to face the older hominid group,” Majumder says. For thousands of
years they remained isolated in the remote islands thus preserving the genetic relics.
Also they show around 2% drop of African alleles, but their apparent physical similarities
with Africans could be due to local adaptation and act of natural selection.

Majumder says the team’s dream would come true if some day
hominid fossils excavated in south-east Asia or Australia show matched sequences
in their ancient DNA with unknown sequences presently found in the inhabitants
of Andaman and Nicobar islands.